One bus commuter, Michael, realised he had been charged the fixed fare on three occasions in the past two months after his bus trip was mysteriously cancelled about 10 seconds after he "touched on".

Trips can be cancelled by tapping a Go Card for a second time on the scanner located at the front of a bus about 10 seconds after the commuter initially touches on.

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But when Michael touched off at the end of his trip, he was charged the $5 fixed fare instead of the normal $4.11 fare, because the system believed he was touching on for another trip for which he would not touch off.

"I wasn't aware that my trip was being cancelled until I checked my online records," said Michael, who requested his last name be withheld.

A spokesman said Translink had contacted Michael and refunded the 89 cents per trip - the difference between the fixed and actual fares.

There was no fault with the Go Card or the smartcard readers, rather the wrong charge was the result of human error.

"Users who touch on or touch off incorrectly receive a message displayed on the card reader next time they use their Go Card," he said.

"Customers with registered Go Cards should check their balance and travel history online and can also request an adjustment."

The spokesman said the current fixed fares "strike the right balance" between tackling fare evasion and not disadvantaging honest commuters.

However, Robert Dow of commuter lobby group Rail Back on Track said it was not uncommon for commuters to be charged a fixed fare without realising.

"It's not the first time we have heard that someone has discovered they have been charged fixed fares on multiple occasions when they get around to checking their Go Card history," he said.

"We believe the bulk of the fixed fares at the moment indicate user errors or system errors."

Mr Dow said other smart cards, including building passes, student identification cards and credit cards, could interfere with Go Card readings, particularly on buses, if the user did not take their card out of their wallet or card holder before touching on or off.

Mr Dow suggested Translink adopt a system recently introduced on London's Oyster Card network, which automatically refunds overpaid charges to commuters who occasionally forget to touch off at the end of their journey.

"If a passenger occasionally fails to touch in and out then the new Oyster Card system will aim to automatically calculate their likely journey and charge the appropriate fare," Mr Dow said.

"The system is routinely scanning for anomalous fixed fares among people travelling every day and the software then makes the necessary adjustments.